This invention is generally directed to processes for treating imaging members such as amorphous silicon, and more specifically, the present invention is directed to a simple direct, economically attractive process for restoring amorphous silicon photoconductive substances with fluorine containing compositions. In one embodiment, the present invention is directed to a process for treating and restoring hydrogenated or halogenated amorphous silicon photoconductive substances including drums with fluorine containing compositions, such as hydrofluoric acid, thereby for example increasing the charge acceptance of these substances. Moreover, in accordance with the process of the present invention amorphous silicon containing undesirable invisible latent scratches on the surface thereof can be treated with fluorine containing substances for the purpose of eliminating the print out of these scratches. Further, in accordance with the process of the present invention image defects which appear as white spots obtained with previously commercially used electrophotographic overcoated amorphous silicon devices can be eliminated. Additionally, by treating amorphous silicon in accordance with the process described hereinafter aging of stored photoconductive drums, which translates into loss of image resolution, can be prevented. Another embodiment of the present invention is specifically directed to restoring used, especially commercially used, amorphous silicon imaging members including drums comprised of amorphous silicon, especially hydrogenated amorphous silicon, and or halogenated amorphous silicon which may contain dopants and contain a protective overcoating layer. The processes of the present invention may also be applicable to the treatment of imaging members comprised of amorphous silicon, including hydrogenated amorphous silicon, and/or halogenated amorphous silicon.
In an embodiment of the present invention, the process comprises restoring hydrogenated or halogenated amorphous silicon imaging members with protective overcoatings by exposing these members to vapors of hydrogen fluoride for an effective time period of, for example, from about 1 minute to about 240 minutes thereby removing the protective overcoating of, for example, silicon carbide, silicon nitride, amorphous carbon, and the like; spray washing the surface of the resulting member; drying the surface; and subsequently depositing a protective overcoating thereby resulting, for example, in a member that when reincorporated into electrophotographic imaging devices enables the achievement of images of increased resolution with substantially no white spots as compared to the member prior to treatment, which is substantially unusable.
Amorphous silicon photoconductors treated in accordance with the process of the present invention are useful as photoconductive imaging members in an electrophotographic imaging or printing apparatus wherein, for example, electrostatic latent images formed on the surface thereof are developed with toner particles, transferred to a suitable substrate such as paper, and subsequently optionally permanently affixed thereto by heat, for example.
Electrophotographic imaging systems, particularly xerographic imaging systems, are well known, and are extensively described in the prior art. In these systems, generally a photoresponsive or photoconductor material is selected for forming the latent electrostatic image thereon. This photoreceptor is generally comprised of a conductive substrate containing on its surface a layer of photoconductive material, and in many instances a thin barrier layer is situated between the substrate and the photoconductive layer to prevent charge injection from the substrate as injection would adversely effect the quality of the resulting image. Examples of known useful photoconductive materials include amorphous selenium, alloys of selenium such as selenium tellurium, selenium arsenic, and the like. Additionally, there can be selected as the photoresponsive imaging member various organic photoconductive materials, including, for example, complexes of trinitrofluorenone and polyvinyl carbazole. Recently, there have been disclosed layered organic photoresponsive devices containing separate charge transport and photogenerating layers. Examples of charge transport layers include various diamines, while useful photogenerating compositions include trigonal selenium, metal and metal-free phthalocyanines, vanadyl phthalocyanines, and the like.
In the patentability search report, there were located the following U.S. Pat. Nos. 4,382,071 directed to a process for preparing silicon tetrafluoride using hydrogen fluorine gas, reference the Abstract of the Disclosure, and note column 1, lines 16 to 18; according to the teachings of this patent, hydrogen fluoride gas is employed as a fluorine source to avoid water in the reaction system and this gas can be introduced into the reaction system at a desired rate and rapidly dissolves in sulfuric acid, which is used as the liquid dispersion medium to react with the dispersed silicon oxide, see column 2, lines 25 to 46; 4,468,443 directed to a process for producing photoconductive members from gaseous silicon compounds, see the Abstract of the Disclosure for example, and note column 2, lines 15 to 54, wherein a photoconductive layer, which comprises substance for formation of such a layer under a gaseous state, is disclosed; as starting substances effectively used for the incorporation of carbon atoms, there can be selected a number of materials, reference column 5; and note the combination compounds represented by the formula B in column 6, beginning at line 35, including SiF.sub.4 ; and 4,849,315 mentioned herein, which relates to processes for restoring halogenated and hydrogenated amorphous silicon imaging members wherein the member subsequent to its utilization in and removal from an electrophotographic imaging apparatus is contacted with a fluorine containing composition, reference the Abstract of the Disclosure.
Also known are amorphous silicon photoconductors, reference for example U.S. Pat. No. 4,265,991. There is disclosed in this patent an electrophotographic photosensitive member containing a substrate, a barrier layer, and a photoconductive overlayer of amorphous silicon containing 10 to 40 atomic percent of hydrogen, and having a thickness of 5 to 80 microns. Additionally, this patent describes several processes for preparing amorphous silicon. In one process, there is prepared, according to the teachings of this patent, an electrophotographic sensitive member by heating the member contained in a chamber to a temperature of 50.degree. C. to 350.degree. C. introducing a gas containing silicon and hydrogen atoms into the chamber causing an electrical discharge in the space of the chamber, followed by depositing amorphous silicon on an electrophotographic substrate at a rate of 0.5 to 100 Angstroms per second. The charge acceptance of these devices is found to be limited by the surface conditions. Further, in some instances as a result of corona interaction with the surface of the devices prepared in accordance with the process of this patent, the surface conductivity undesirably increases resulting in a loss of image resolution, and a decrease in image density within less than about 1,000 imaging cycles. Accordingly, while the amorphous silicon photosensitive devices of the '991 patent are useful, their selection as a commercial device for a number of imaging cycles is not readily achievable. Also known are the aforementioned members with protective overcoatings of silicon nitride, silicon carbide, amorphous carbon, including hydrogenated, 10 to 45 atomic percent for example, and the like. These members, especially when selected for use in commercial electrophotographic, especially xerograpahic, imaging apparatus are generally only useful for about 300,000 copies in most instances; thereafter it is the present practice to discard these members or drums and replace them with new members at a substantial cost. With the processes of the present invention, the used imaging members, including drums of amorphous silicon with protective overcoatings, can be restored and reused thereby enabling a substantial cost savings of about $500 or more per drum in an embodiment of the present invention. Generally, the restoration process of the present invention results in a cost of about $30 per drum as compared to the costs of new drums of $500 or more.
In Xerox U.S. Pat. No. 4,849,315, the disclosure of which is totally incorporated herein by reference, there is illustrated an improved process, which comprises (1) providing an amorphous silicon photoconductive substance, and (2) contacting this substance with compositions containing fluorine for a sufficient period of time to enable an increase in the charge acceptance of the photoconductive substance, and/or the elimination of surface scratches contained thereon by, for example, etching away the old surface and reforming a new surface. In one specific embodiment of the aforementioned patent, there is provided an improved process for treating amorphous silicon photoconductive materials, which comprises (1) providing a virgin unused amorphous silicon photoconductive substance, and (2) contacting this substance with vapors generated from hydrofluoric acid for a period of time of from about one minute to about 240 minutes, and preferably from about 10 minutes to about 60 minutes, wherein there results an amorphous silicon photoconductor which has increased charge acceptance and resolution as compared to amorphous silicon not treated with hydrofluoric acid vapors.
In a further specific embodiment of the aforementioned patent, there is provided an improved process for restoring hydrogenated or halogenated amorphous silicon imaging members, which comprises (1) providing a hydrogenated or halogenated amorphous silicon photoconductive member subsequent to its utilization in and removal from an electrophotographic imaging apparatus, and (2) contacting this member with vapors generated from hydrofluoric acid for a period of time of from about one minute to about 240 minutes, and preferably from about 15 minutes to about 60 minutes wherein the charge acceptance of the removed member has increased, and/or surface scratches contained therein have been substantially eliminated.
Illustrated in U.S. Pat. No. 4,357,179, the disclosure of which is totally incorporated herein by reference, is a method for preparing devices containing high density amorphous silicon or germanium wherein with respect to the amorphous silicon, n-doping or p-doping can be affected with dopants such as phosphorous or boron, reference the disclosure in column 5, beginning at line 44, and specifically at lines 55 to 65.
Also, in U.S. Pat. No. 4,237,150 there is disclosed a method for preparing hydrogenated amorphous silicon wherein ammonia is introduced into a reaction chamber with a silane gas for the purpose of enhancing the photoconductivity of the resulting hydrogenated amorphous silicon films, reference the disclosure in column 1, beginning at around line 20.
Further, U.S. Pat. No. 4,237,151 discloses the preparation of hydrogenated amorphous silicon substances by thermally decomposing silane or other gases at elevated temperatures and under specific vacuum conditions wherein a gaseous mixture of atomic hydrogen and atomic silicon result, followed by depositing this mixture onto a substrate situated outside a heated tungsten tube wherein a film of hydrogenated amorphous silicon is formed on the substrate. In column 4, beginning at line 58, it is indicated that conventional doping gases can be added to the silane if desired.
Furthermore, U.S. Pat. No. 4,356,246 describes a specific noncrystalline silicon powder having excellent photoconductivity comprised of silicon and hydrogen, which powder exhibits specific characteristics such as an infrared absorption spectrum characterized by absorption peeks centered about certain areas, reference the Abstract of the Disclosure. There is disclosed in column 5, beginning at line 41, that in addition to hydrogen, other elements such as oxygen, fluorine, chlorine, bromine, iodine, phosphorous, boron, solely or in the form of a combination, may be contained in the amorphous silicon particles for the purpose of controlling the electrical conductivity thereof.
Additionally, U.S. Pat. No. 4,361,638 discloses a light sensitive electrophotographic element including a photoconductive layer comprised of amorphous silicon and a carbon based material doped with hydrogen and fluorine, reference the disclosure in column 4, beginning at line 3.
Furthermore, U.S. Pat. No. 4,365,013, the disclosure of which is totally incorporated herein by reference, illustrates an amorphous silicon layer, which apparently can be rendered highly photoconductive by doping with hydrogen, or by doping with impurities, reference the disclosure in column 2, beginning at line 46. Doping materials disclosed include halogens such as fluorine, chlorine, bromine and iodine, reference the disclosure in column 2, beginning at line 57.
Other prior art include U.S. Pat. Nos. 4,342,044; 4,394,426; 4,468,443 and 4,490,208. In the '044 patent, reference column 4, beginning at line 54, and particularly at line 65, there is disclosed that the nonoptimum spectral response of the prior art amorphous silicon photoresponsive devices is overcome by adding one or more band gap adjusting elements to the amorphous photoresponsive alloy. More specifically, as stated at line 65, "The amorphous alloy incorporates at least one density of states reducing element, fluorine. The compensating or altering element, fluorine or elements, can be added during deposition or thereafter.". Also, disclosed in the '208 patent are methods of obtaining thin films of silicon by doping a p or n type thin film of silicon with an impurity element under a plasma discharge of gas of at least one element selected from among fluorine, chlorine, bromine, iodine, and hydrogen, reference column 1, lines 51 to 63. In the '426 patent, there is disclosed a photoconductive member containing amorphous silicon atoms as a matrix, hydrogen or halogen atoms; and as an intermediate layer an amorphous material containing, for example, silicon atoms and nitrogen atoms.
Additionally, described in U.S. Pat. No. 4,634,647, the disclosure of which is totally incorporated herein by reference, are processes for preparing amorphous silicon, which is useful for incorporation into an electrophotographic imaging apparatus, wherein images of high resolution can be obtained for a number of imaging cycles. This process involves simultaneously treating amorphous silicon with dopants such as boron and phosphorus.
Substantial interest has been directed to obtaining amorphous silicon photoreceptor materials since they possess a number of advantages in comparison to, for example, amorphous selenium based materials in that amorphous silicon is of extreme hardness and will not crystallize or wherein crystallization is minimized over extended time periods even at temperatures as high as several hundred degrees Centigrade. Additionally, especially hydrogenated or halogenated amorphous silicon photoconductors have excellent photoelectronic properties, high absorption coefficients through the visible region, and are relatively low in useful life cost in comparison to selenium photoconductors. Moreover, amorphous silicon photoreceptors are capable of ambipolarity as they can be xerographically charged, and discharged either positively or negatively in various imaging systems. Furthermore, amorphous silicon can be modified by adding various dopants thereto such as boron and phosphorus, enabling this material to function as a p or n type semiconductor device; and amorphous silicon may be alloyed with other materials including germanium and tin for the purpose of providing a material which will be photosensitive in the infrared region of the spectrum. Moreover, amorphous silicon materials are inert and nontoxic rendering them highly desirable as a photoconductive imaging member.
While processes are known for preparing amorphous silicon with protective overcoatings, in some instances these processes result in members which, after repeated usage in electrophotographic imaging devices, have decreased charge acceptance causing the resulting images to be of poor density. In addition, the images have scratch printout. Scratches, both latent and visible can be produced as a result of handling, mechanical interaction with the developer carrier beads or the cleaning system. While it is not desired to be limited by theory, it is believed that as a result of a continuous mechanical interaction of developer carrier beads with the protective surface of the amorphous silicon, undesirable scratches result thereon which if deep, for example, about one micron, can scratch through the protective overcoat of, for example, silicon carbide or nitride. The silicon-hydrogen and/or silicon-silicon bonds in the scratched regions are broken, and in the presence of water vapor are reformed into silicon-hydroxy type bonds. The charge acceptance in the regions of scratch, which is limited by the surface conditions, decreases as a result of this mechanical interaction causing an undesirable reduction in the density of the images in the scratched regions. Further, the mechanical damage caused by the developer beads during the development step lowers the charge acceptance of the entire amorphous silicon photoresponsive device or drum, and causes a corresponding decrease in the density of the total image.
Another type of mechanical damage observable with many overcoated hydrogenated or halogenated amorphous silicon imaging members is confined to small isolated areas of the surface; this damage also generally being referred to as scratches. These scratches are caused, for example, during xerographic imaging processes wherein the amorphous silicon photoconductor is subjected to cleaning with wiper blades. Furthermore, interaction of the amorphous silicon with isolated carrier particles contained in the developer mixture can cause scratches. Additionally, these scratches can be generated during the handling of an amorphous silicon (hydrogenated or halogenated) drum while it is being manufactured, and during the positioning of this drum within the machine involved. These latent scratches, which are not visible, damage the surface of the amorphous silicon causing a reduction in the charge acceptance thereof, and a print out of the scratches. Thus, subsequent to a minimum number of imaging cycles mechanical interaction of amorphous silicon member with carrier beads causes a reduction in image density wherein scratches print out. Eventually, the amorphous silicon photoreceptor, which was initially selected for its durability, may no longer be useful, and thus is discarded.
Additionally, it is believed that the degradation of the electrophotographic performance of amorphous silicon is caused by the sensitivity of the silicon imaging device to chemical alterations by exposure to a corona atmosphere, especially at high humidities. These sensitivities create fundamental limitations for the practical use of devices wherein the exposed surface contains substantially amorphous silicon. While this problem can be minimized by encapsulating the silicon with a chemically passive hard overcoating of amorphous silicon nitride, amorphous silicon carbide, or amorphous carbon, such devices when incorporated into xerographic imaging systems can result in image blurring and very rapid image deletion in a few imaging cycles, typically less than about ten. Furthermore, in these overcoated devices poor image quality with cycling is caused by an increase in the surface conductivity of the underlying amorphous silicon layer, rather than to abrasion or chemical interaction as occurs with amorphous silicon containing no protective overcoating layer. This conductivity increase is induced by the electric field existing at the surface of the overcoated device, similar to the effect resulting from the field effect in well known metal-insulator-semiconductor devices, causing a lateral spreading of the photogenerated charges in the fringe electric fields associated with line or edge images projected on the photoreceptor surface thereby resulting in undesirable image blurring and image deletion. The composition of the silicon nitride or silicon carbide has to be optimized to prevent this image blur. The material required to prevent image blur is non-stoichiometric and is not mechanically as durable, and is likely to wear off in 300,000 copies as a result of cleaning action. Also, the thickness of the overcoat in the virgin device is determined by the residual potential arising from the overcoat. Therefore, the protective overcoat thickness is limited to less than one micron. The aformentioned amorphous silicon drums must thus be replaced after about 300,000 copies at substantial cost. This disadvantage is avoided with the processes of the present invention in that the used drums can be restored and commercially reused.
Accordingly, there continues to be a need for obtaining amorphous silicon imaging members which can be repeatedly used in a number of imaging cycles. Additionally, there continues to be a need for processes for treating amorphous silicon photoconductive materials for the purpose of increasing charge acceptance and resolution thereof, and eliminating surface scratches and white spots therefrom. Moreover, there continues to be a need for restoring overcoated amorphous silicon photoconductive materials thereby resulting in the elimination of surface scratches and an increase in charge acceptance for these materials, and enabling imaging members, especially commercially used drums, to be restored rather than be disposed of. Further, there continues to be a need for processes wherein commercially used amorphous silicon photoconductive members can be treated primarily for the purpose of increasing the charge acceptance thereof. Also, there continues to be a need for direct economical processes for restoring overcoated amorphous silicon imaging members, which subsequent to restoration can be selected for incorporation into an electrophotographic imaging system, and wherein there results in many instances increased charge acceptance allowing the restored member to be useful for a substantial number of imaging cycles without causing a degradation in image quality, and specifically without resulting in images of low density and poor resolution. Furthermore, there continues to be a need for processes to refurbish or rejuvenate overcoated amorphous silicon drums that show loss of density and scratch print out subsequent to use in an imaging apparatus.